API: Use 'submodule' param type, when available

This allows action=paraminfo to indicate which parameters are
submodules, and will allow ApiSandbox and HTMLized action=help to
properly support action=flow.

There are no ordering requirements between this and the related core
change.

Change-Id: Id4b85f0a719624c96014b136325c362fc5f72e2b
Related-To: Id31babdc81d970ac781ec11daa3cdafef18ecd5d
Merge "API: Use 'submodule' param type, when available"